This invention relates to the plating with metal of a surface of a filled polyamide thermoplastic such as nylon-6.
In the automotive, home appliance, electronic, container and many other industries there is a great demand for functional and/or decorative metal plating of filled thermoplastic articles. Typically these articles are electroplated with copper, nickel, chromium, and combinations thereof to provide a functional and/or decorative metallic surface. To enable electroplating, the plastic surface must be treated to make it electrically conductive and to promote adhesion to it of electrodeposited metal. To promote adhesion the surface is usually roughened either by mechanical abrasion or chemical etching. The surface is made electrically conductive by electroless plating of copper, nickel, or other conductive metals.
U.S. Pat. No. 4,349,421 discloses a method of plating a polyamide thermoplastic filled with very fine particles of wollastonite such as nylon-6. To promote adhesion, a portion of the filler particles in the surface are dissolved and removed by a dilute acid such as hydrochloric acid. The dilute acid does not attack or etch the nylon resin, but rather only removes a portion of the filler to produce a roughened surface. The roughened surface is made electrically conductive by conventional electroless plating with copper or nickel. Thereafter, the desired metal surface is produced by conventional electroplating of copper, nickel and chromium on the conductive surface.
According to this patent, electroplated filled particles produced by this process have an adhesion or peel strength of about 3 to 6 pounds per linear inch. However, in practice the results of this process are erratic and inconsistent, and consistent adhesion has not been obtained with this process. For no apparent reason adhesion to electroplated nylon-6 articles has varied from almost zero to more than six pounds per linear inch. Moreover, articles chrome plated by this process frequently have visually observable and aesthetically unacceptable streaks or variations in the color, appearance and reflectivity of the finished plated surface.